This application claims the priority of German patent document 102007050716.1-35, filed Oct. 22, 2007, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for improving integrity communication in a satellite navigation system.
Global Navigation Satellite Systems (GNSS) (sometimes referred to herein as “satellite navigation systems”) are used for position determination and navigation on the ground, on water and in the air. GNSS Systems, such as, for example, the European Navigation Satellite System currently being constructed (also referred to herein as the Galileo System, or simply “Galileo”) include i) a satellite system (space segment) comprising a plurality of satellites, ii) an earth-fixed receiving device system (ground segment), which comprises several ground stations as well as Galileo sensor stations and is connected with a central computing station, and iii) utilization systems (users), which evaluate and use the satellite signals transmitted by radio from the satellites, particularly for navigation. The arrangement and the method can also be used for regional satellite navigation expansion systems or regional satellite navigation systems as well as for regional navigation systems.
In a GNSS, precise detection of a user's position requires local as well as global integrity. Integrity means especially that, on the one hand, the GNSS is capable of warning a user within a certain time period when parts of the GNSS should not be used for navigation, for example, in the event of a failure of system components, and that, on the other hand, the user can trust the navigation data which he receives by way of satellite navigation signals from the satellites of the GNSS, particularly that he can rely on the precision of the received navigation data.
In the integrity concept of Galileo, it is planned to monitor each satellite from the earth-fixed receiving device system and to transmit corresponding message signals with respect to the behavior of each satellite to use systems; for example, an estimated signal-in-space accuracy (SISA) of a satellite or a simple error indication “Not OK” in the event of a faulty satellite; or the precision with which errors on the navigation signals can be determined by the observation system.
Galileo should also be capable of monitoring the signal-in-space (SIS) within the ground segment by using the measurements from the individual Galileo sensor stations. Based on the known positions of the Galileo sensor stations, the current position of a satellite and thereby the maximal error of the satellite or of the signal in space emitted by it (the so-called signal-in-space error, SISE) can be estimated.
A prediction of the distribution of the SISE can be represented by a normal statistical distribution with the smallest standard deviation. This prediction is called signal-in-space accuracy (SISA). By means of the SISA, the difference can be described between the current 4-dimensional position (orbit and clock time) of a satellite and the predicted 4-dimensional position that is contained in a navigation message.
However, the estimation of the SISE is itself an error-laden process. As a rule, it is therefore assumed that the distribution of the current SISE around the value of the estimated SISE can be described by a normal statistical distribution with the standard deviation, which is called the signal-in-space monitoring accuracy (SISMA). The SISMA therefore is the precision of the estimation of the SISE for a satellite.
In the case of the previous concept of Galileo for the transmission of the SISMA, for each satellite a scalar SISMA value is transmitted that is conservative for every conceivable position of a use system (user position). However, as a result, much of the efficiency of the GNSS is given away because a clearly excessive SISMA value is transmitted in many positions, which results in a high-expenditure integrity communication in the GNSS, and much of the efficiency of the observation system is not made accessible to the user.
Since the individual observation stations have a relatively high failure probability, it will, in addition, again be necessary to take into account possible failures of ground stations in advance when calculating the scalar value. Thus, a sufficiently large number of failures must be considered such that even the strictest continuity demands can be met. However, for use systems that do not make such high demands on the continuity, this consideration will again result in a clearly excessive scalar value. In addition, for computing the scalar value for each satellite, the least demanding observation station is not used, which is clearly more conservative than is frequently necessary.
It is therefore an object of the present invention to provide a method and apparatus for improving integrity communication in a satellite navigation system.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which, for the different observation stations of a satellite navigation system (or for groups of observation stations of a satellite navigation system), error budgets are transmitted to use systems, rather than the scalar SISMA value referred to previously. In particular, such error budgets can be transmitted as ranges of possible values for the average of several error budgets, where the average is taken over the individual error budgets.
The error budget p(x), as provided herein, is described by defining two functions qr(x) and ql(x), to which the following expressions apply:
                              ∫                      -            ∞                    L                ⁢                              p            ⁡                          (              x              )                                ⁢                      ⅆ            x                              ≤                        ∫                      -            ∞                    L                ⁢                              ql            ⁡                          (              x              )                                ⁢                      ⅆ            x                                ,    and                      1        -                              ∫            L            ∞                    ⁢                                    p              ⁡                              (                x                )                                      ⁢                          ⅆ              x                                          ≥              1        -                              ∫            L            ∞                    ⁢                                    qr              ⁡                              (                x                )                                      ⁢                          ⅆ              x                                            ,  for all values of L. That is, the above relationships must be satisfied for all values of the variable L. A description of ql and qr is also transmitted. The functions ql and qr are multiples of normal statistical distributions, which may have different standard deviations and average values.
Based on this information, and based on the positions of the observation stations (also transmitted to the use systems) and the information concerning the momentarily used observation stations, particularly of individual use systems (also transmitted to the use systems), an estimate of the distribution can be calculated which indicates the precision of the error estimate regarding generation of the navigation signal. As a result, the estimates for the distribution of the precision of the error estimate for the generation of the navigation signal (which some use system are using) can be more precise (narrower, lower) because the estimate for the distribution of the precision of the error estimate for the generation of the navigation signal can be computed by a use system, as a function of its location.
Thus, an estimate for the distribution of the precision of the error estimate for the generation of navigation signal no longer needs to be calculated such that it is valid for all use systems in a center of the satellite navigation system, and transmitted to the use systems. Rather, because the computation of the estimate for the distribution of the precision of the error estimate for the generation of the navigation signal is performed in each use system based on its location, continuity demands of individual use systems can also be taken into account, so that the highest demands on the continuity need no longer be met at each use system. On the whole, as a result of the invention, the integrity communication in a satellite navigation system can be improved significantly because clearly more precise estimates for the distribution of the precision of the error estimation of the generating of the navigation signal can be used in the use system.
The estimate for the distribution of the precision of the error estimate for generation of the navigation signal may also be a scalar value.
According to an embodiment of the invention, a method is provided for improving integrity communication in a satellite navigation system which comprises i) a space segment having several satellites that emit navigation signals for the reception and evaluation by use systems for position determination, and ii) a ground segment that includes several observation stations, which monitor the satellites. The method according to the invention comprises the following:                transmitting, with a navigation signal of a satellite, an error budget for different observation stations or groups of observation stations;        transmitting the positions of the observation stations;        transmitting information regarding which observation stations are supplying observations;        transmitting the elevation angle at which observation stations supply observations;        transmitting information regarding which ground stations are supplying no observations;        transmitting information regarding which observations from specific ground stations to specific satellites are not available, although they should be available according to the preceding rules; and        reception of the navigation signal and evaluation of the error budget contained therein.        
Based on the error budget and the coordinates of the observation stations used, the observation system computes a signal-specific, user-system-dependent and user-system-position-dependent estimate for the distribution of the precision of the error estimate for generation of the navigation signal, which indicates the precision of the error estimate for generation of the navigation signal by the observation system.
In this case, generation of the navigation signal relates particularly to the orbit of the satellite emitting the signal, the transmission time of the signal and the signal structure. As a result of this method, a use system can receive significantly smaller values than the initially explained conservative (and therefore relatively large) values. On the whole, integrity communication in the satellite navigation system to individual user systems is therefore improved. The transmission can generally take place at a relative low repetition rate in order to minimize the data traffic in the satellite navigation system attributable to integrity communication.
According to an embodiment of the invention, the error budget can be transmitted as a statistical value.
Furthermore, according to an embodiment of the invention, the error budget may also be transmitted as intervals for the average values of several error budgets.
According to an embodiment of the invention, the error budget may also be a function of elevation. This permits a still more precise computation of a scalar value, and thus improves the precision of the error estimate.
Furthermore, according to an embodiment of the invention, coordinates of the observation stations that are used to determine the error budgets and the shading angles associated therewith, can be transmitted, and can be used to compute of the distribution of the precision of the error estimate for generation of the navigation signal. Here also, the transmission can take place at a relatively low rate of repetition.
In addition, according to an embodiment of the invention, the expected continuity of the availability of measurements of each observation station used for the determination of the error budget (or of each group of observation stations) can be transmitted and used for the computation of the scalar value. This technique proves advantageous mainly when computing the distribution of the precision of the error estimate for generation of the navigation signal, and its reliability.
In a further embodiment of the invention, a first alarm message can be transmitted with the navigation signal when an observation station has failed. A use system can thereby be warned, and such warning can be taken into account when computing the distribution of the precision of the error estimate for generation of the navigation signal on the basis of the received error budget.
Furthermore, according to an embodiment of the invention, if a measurement of a satellite signal by an observation station is lost, the number of the observation station and of the concerned satellite are transmitted with the satellite navigation signal, so that a use system can detect which observation stations should not be used for computation of the distribution of the precision of the error estimate for the generation of the navigation signal of a specific (satellite) signal.
In addition, according to an embodiment of the invention, a second alarm message can be transmitted with the navigation signal when the error budget is to be changed. Use systems can thereby be informed that a new computation of the distribution of the precision of the error estimate for the generation of the navigation signal may be necessary on the basis of the changed error budget.
In a further embodiment, the invention provides a use system for a satellite navigation system, particularly a mobile navigation device, which is constructed to be used with a method according to the invention and is constructed as described above.
In particular, according to an embodiment of the invention, the use system may also be constructed for computing the distributions of the precision of the error estimate for generation of the navigation signal based on received error budget, and for determining an integrity risk therefrom.
Furthermore, according to an embodiment of the invention, the use system may also be configured to compute the distribution of the precision of the error estimate for the generation of the navigation signal such that a predetermined continuity can be reached. Corresponding to its continuity demands, a use system can thereby compute a correspondingly optimized distribution of the precision of the error estimate for generation of the navigation signal. The optimization takes place in that, even in advance, observation stations are taken into account as potentially not available. This optimization can be carried out in the user system in a manner adapted to the individual user demands.
Finally, according to an embodiment of the invention, an arrangement is provided for improving integrity communication in a satellite navigation system, wherein the satellite navigation system comprises i) a space segment with a plurality of satellites which emit navigation signals for the reception and evaluation by use systems for the position determination, and ii) a ground segment with several observation stations which monitor the satellites. The arrangement comprises:                error budget determination devices for determining an error budget for different observation stations (or groups of observation stations); and        transmission devices for transmitting the error budget to at least one satellite for emission with a navigation signal of a satellite.        
Such an arrangement can be situated, for example, in the ground segment and can influence integrity communication in the satellite navigation system, by means of the error budget or budgets.
The terms used in the attached list of reference symbols and the assigned reference numbers are used in the description, in the claims, in the abstract and in the drawings.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.